Overflow trunking system



C. E. LOMAX OVERFLOW TRUNKING SYSTEM Filed Do. e, 1921 2 Sheets-Sheet 1 I YImff Elaranne Elumsx A F M M C. E. LOMAX OVERFLOW TRUNKING SYSTEM Filed Dec. 6, 1921.

.ElErauca Blames: Z. 511- r:

Y Inusnimr Patented Dec. 9 1924.

UNIT-ED STATES CLARENCE E. LOMAX,

COMPANY, OF CHICAGO,

OF CHICAGO, ILLINOIS, A SSIGNOB 'IO AUTOMATIC ELECTRIC ILLINOIS, A CORPORATION OF ILLINOIS.

OVERFLOW TBUNKING SYSTEM.

Application filed December a, 1921. Serial no. 520,406;

To all whom it may concern:

Be it known that I, CLARENCE E. LOMAX, a citizen of the United States of America, and

a resident of Chicago, Cook County, and I State of Illinois, have invented certain new and useful Improvements in Overflow Trunking Systems, of which the following is a specification.

The present invention relates to overflow trunking arrangements for telephone systems in general, but is concerned more particularly with overflow trunking arrangements for automatic telephone systems; and the principal object is to provide a new and improved overflow trunking system, in which the trunks are regrouped and rearranged so as to be moreeiiicient.

In order to explain this object a little more fully, it may be said that it has not proven to be practicable to provide automatic switches which have access to more than a given number of trunks of a particular group, which number is in some types of switches as low as ten and in others as high as twenty-five. Now, when there are a great number of trunks, forty or fifty for example, in a group, they must be divided into subgroups, the number of which will depend upon the number of trunks to which the automatic switches that are used have access. Assuming that the automatic switches in question are selectors of the well-known vertical and rotary type, have access to ten trunks each, on any one level, and accordingly the major group trunks may be divided into a number of subgroups comprising ten trunks each. It has been found, however, that when this is done some sub-groups become busy while some of the others still have trunks available, and accordingly the group as a whole would be more eflicient if the size of each of the subgroups could be increased so as to include more trunks. This has been accomplished heretofore by replacing each sub-grou of ten trunks with a smaller number 0 individual trunks. The contacts thus left available, which in all cases are the last ones to be tested, are all multiplied together throughout the various groups and connected to one common group of trunks. The number of trunks individual to each roup is made large enough to carry practically all the traffic from that sub-group, but is small enough so that when there is an extra they will ordinarily f the various groups ofswitches, and in the wave of traflic they will all be taken into use, and the combined overflow from all the dif- 1 ferent groupsis taken care of by the one common group of trunks. Since, as before intimated, all the individual trunks must be busy before any of the common trunks are taken for use, the several sub-grou s are combined into one larger and more e cient group. This arrangement, while in some cases satisfactory, is in other cases objectionable because of its inflexibility. For example: When this arrangement is used the trunks cannot be given the usual slip be tween shelves, for if they are, the last trunk on a particular shelf becomes the first on the next shelf, and consequently the common trunks are used up. along with the individual trunks, indiscriminately. There is still another situation which is very commonly met up with in which this scheme fails to meet the demand. This situation arises when switches, such as rotary line switches of the well-known type, in which the wipers have no normal position are used to select a group of trunks. In this case it is perfectly obvious that the switch, since it does not return to normal position but rests on the last trunk used, is as likely to take one of the common trunks for use as it is to take one of the individual trunks, and consequently all the advantages of such an arrangement are lost when switches of this type are used. In order to overcome this difficulty, both in the case of selectors with slipped banks between case of switches in which the wipers have no normal position, the individual trunks and the common trunks are provided in my improved system in the usual manner with the" exception that the common trunks are normally made busy and arrangements are made such that, as soon as all the trunks individual to any group of switches become busy, the common trunks are rendered idle only to the switches in that group. After the common trunks have been made idle to a particular group of switches they are made busy to that group of switches again as soon as one of the trunks individual to that group becomes idle.

In addition to the foregoing, the invention includes a novel trunking arrangement for handling traffic between two different switching points or exchanges, using both one-way and two-way trunks. The two-way g given exchange only when the one-way trunks accessible to such switches are in use.

Referring now to the accompanying drawings comprising Figs. 1 and 2, they show by means of the usual circuit diagrams a suflicient amount of the apparatus in a system embodying the principles of the invention to allow the invention to be thoroughly understood. F 'ig. 1 shows two subscribers line switches G and C, together with a selector D to which they have common access.

Fig. 2 shows a two-way trunk line betweentwo automatic exchanges which is accessible to the selector D, Fig. 1. When this trunk line is seized in either exchange, by an automatic switch which connectswith either the repeater E orthe repeater E, depending upon which exchange the seizing switch is in, the connection is extended through the distant exchange byway of the normally Iconnected selector D or D, as the case may The line switches C and C, Fig. 1, are mechanically of the well-known rotary type in which the wipers have no normal position and move in a forward direction only.

The selectors D, D, and D are of the well-known vertical and rotary type and accordingly have their bank contacts arranged in horizontal rows or levels. The circuits of these three selectors are identical with'each other and will be explained fully hereinafter.

Therepeaters E and E are exactly alike and are provided primarily to avoid the necessity of having three conductors in the associated inter-exchange trunk line.

For each of the trunks individual to the group of switches in which the line switch 0 is located, there is a so-called chain relay which is energized whenever the associ'ated trunk is busy. .One of these chain relays is the relay 35, Fig. 1, and another is the relay 36. The normally open contacts controlled by these relays are connected in series. Accordingly, when all of these chain relays are energized, indicating that all the trunks individual to this group are busy, ground is extended to the conductor 39 which, as is indicated on the drawings, is a common conductor that connects with the lower winding of each of the relays such as relay 40, of which one is provided at each group of switches for each of the over-flow trunks. The similarly located relays at the group at which the line switch C is located are connected up in the same manner and likewise the .relays associated with the trunks extending from the indicated level of the selector D.

The system, having been explained generally, will now be explained in connection with a detailed description of the operation of the apparatus shown. For this purpose it will be assumed that the subscriber, whose line terminates in the line switch C, desires to make a call. It will be assumed further that all the trunks individual to the group in which the line switch 0 is located are busy, and consequently that all the chain relays such as chain relays 35 and 36 are energiized. That being the case, there is a ground potential upon conductor 39 by way of armature 37 and its working contact, intervening armatures and contacts, and armature 38 and is iworking contact and all the relays suc as the relay 40, associated with t e common trunks are energiized and the associated conductors such as t e conductor 29 are disconnected from ground and are extended through to the switches ahead. However, while the common trunks have been made accessible to the group of line switches in which the line\ switch C is located, they may still be busy to the line switches in the group in which the line switch C is located, depending upon whether or not all the trunks individual to that group are busy. When the receiver is removed at the calling substation, a circuit is closed over line conductors 11 and 12 for line relay 15 of the line switch C. Line relay 15, upon energizing, places ground upon private normal conductor 13, thereby causing the calling line to test busy to the switches having access to it and closing a circuit for switching relay14 and stepping magnet 16 in series; and at" armature 20 connects test wiper 23 to the above circuit at' a point between switching relay 14 and stepping magnet 16. In response to the closure of these circuits the line switch C operates to select an idle trunk in the usual manner. Assuming that the trunk line comprising conductors 28-30, inclusive, is the first one found to be idle, stepping magnet 16 does not energize again when the wipers 2224, inclusive, arrive upon the bank contacts, 2527, inclusive, and switching relay 14, which heretofore has been short circuited by the ground potential which is present on other busy test contacts, energizes in series with stepping magnet 16. Stepping magnet 16, however, does not energize in series with switching relay 14 on account of the high resistance of the latter. Switching relay 14, upon energizing, opens the test circuit and prepares the holding circuit at armature 18, and disconnects the line conductors 11 and 12 from the winding of line relay 15 and ground respectively, and extends them by way of armatures 17 and 19, wipers 22 and 24, bank contacts 25 and 27, conductors 28 and 30, and armatures 62 and 65 and their resting contacts to the windings of line relay 45 of selector D. Line relay 45, upon energizing over the calling subscribers loop,

closes a circuit for release relay 46'at armature 55. Releaserelay 46, upon energizing, opens a point in the circuit of release magnet and prepares a circuit for vertical magnet 52 at armature 57, and at armature 56 places ground upon release trunk conductor 29, thereby establishin the usual holding circuit for switching re ay 14 of line switch C before the slow-acting line relay 15 has had time to deenergize. This holding circuit, it may be noted, includes release trunk conductor 29, the working contact of armature 42 and said armature, and conductor 29'. When ground is placed upon conductor 29, a circuit is closed for the upper winding of relay 40 so as to hold the said relay energized independent of its lower winding.

To continue with the operation of the selector D, the calling subscriber may now operate his calling device in accordance with the first digit of the desired number, thereby producing a corresponding number of interruptions in the circuit of line relay 45. Each time line relay 45 deenergizes in response to one of these interruptions it closes a circuit for vertical magnet 52, which circuit includes armature and its resting contact, armature 57 and its working contact and relay 47. By the operation of vertical magnet 52, the shaft and wipers of selector D are raised step by step until they stand opposite the desired level of bank con tacts. Relay 47 is energized in series with vertical magnet 52 and being slow-acting retains its armature attracted throughout the vertical movement and, upon-the closure of off-normal contacts 59, completes a circuit for stepping rel'ay 48 as follows: From ground by way of the working contact of armature 56 and said armature, armature 58 and its working cont'act, interrupter contacts 54 of rotary magnet 53, off-normal contacts 59, and stepping relay 48 to battery. Stepping relay 48, upon energizing, pre- .pares a circuit for rotary magnet 53 at armature 60 and at armature 61 closes a looking circuit for itself. At the end of the vertical movement slow-acting relay 47 deenergizes and completes the circuit of rotary magnet 53. Rotary magnet 53, upon energizing, advances the wipers 66-68, inclusive, into engagement with the first set of contacts in the level opposite which they are raised, and at its interrupter contacts 54 opens the circuit of stepping relay 48. Stepping rel'ay 48, upon deenergizing, opens the circuit of rotary magnet 53 at armature 60, whereupon said rotary magnet deener- 'izes' and closes its interrupter contacts again.

From this point the operation depends upon whether the trunk line terminating in the first set of contacts is busy or idle. If it is busy, there is a ground potential upon the test contact thereof, and stepping relay 48 is energized again, whereupon the wipers are stepped into engagement with the next set of contacts. s operation continues until an idle or ungrounded test contact is reached.

' Before oing further it will be said that the trunk ine which terminates in b'ank contacts 69, 70, and 71 and which leads to the repeater E is normall made busy, as shown, and that there m'ay iie other similar trunk lines also normally made busy terminating in the same level of contacts of the selector D. Other contacts, however, of which two sets are shown, are connected in the usu'al manner to trunks which extend by way of the usual repeaters to selectors in the distant oflice. It may be said also that the three conductor trunk associated with the repeater E on the distant end of the twoway trunk line is also normally made busy in the same manner as the trunk associated with the repeater E and that it terminates in the banks of selector switches in which there are terminated other trunks associated with slmilar repeaters, and also some trunks associated with one-way repeaters, which repeat from the distant ofiice over a two-conductor trunk line to selectors located in the oflice in which the selector D is located.

It will be assumed that all of the one-way trunks accessible from the selector D are in use, and consequently that all the chain relays, such as the chain relays 80 and 81, are energized. That being the case, a ground potential is placed uponv conductor 84 by way of armature 82 and its working contact, intervening armatures and contacts, and armature 83 and its working contact. As a result, relay 87, as well as the similar relays associated with the other two-way trunks, is energized. With this condition existing, and assuming that the trunk line terminating in bank contacts 69-7 1, inclusive, is the first one found to be idle, the wipers of the switch D stop when these contacts are reached, and switching relay 49, which heretofore has been short circuited by the ground potential which is present on the busy test contacts, energizes in series with stepping relay 48. Stepping relay 48, however, does not energize in series with switching relay 49 on account of the high resist ance of the latter. Switching relay 49, upon energizing, opens the test circuit and prepares the holding circuit at armature 63; removes ground from the armature of line relay 45 at armature 64; and at armatures 62 and 65 disconnects conductors 28 and 30 from the windings of line relay 45 and extends them by way of wipers 66 and 68, bank contacts 69 and 71, and conductors 72 and 74 to the upper and lower windings of line relay 100 of the repeater E. Line relay 100, upon energizing over the calling subscribers loop, closes a c1rcuit for release relay 101 at armature 105. Release relay 101, upon enertrunk conductor 73. a circuit is completed for the upper winding of relay 87.' Thi is done so as to hold the said relay energized independent of its lower winding.

Returning now to'the repeater E, as a further result of the energization of release relay 101, it closes a circult for relay 104 at armature 108. Relay 104, upon energizing, disconnects trunk conductors 113 and 114 from the selector D at armature 111 and 112 and connects them to a closed bridge including the working contact of armature 106 and said armature, impedance 103, and the resting contact of armature 110 and said armature. In response to the closure of this bridge, line rela nected selector 2 on the distant end of the trunk line energizes and closes the circuit of release relay 133. Release relay 133, upon energizing, prepares the switch for operation at armature 134 and at armature 135 places ground upon release trunk conductor 131, which conductor extends by way of the r er E to the bank contacts from which the latter is accessible,- thereby guarding the trunk against seizure in the usual manner.

The calling subscriber may now operate his calling device in accordance with the second digit of the desired number, thereby producing acorresponding number of interruptions in the circuit of line relay 100 of repeater E. Each time line relay 100 deenergizes in response to one of these-interruptions, it opens the circuit of line relay 129 of the selector D at armature 106, and at armature 105 closes the circuit of slowacting relay 102. Relay 102 is accordingly maintained energized through the series of interruptions, and at armature 110 completes the bridge across conductors 113 and 114 by the more direct path which excludes impedance 103. This is the usual expedient made use of in order to obtain first-class switch control. I

The selector D in response to the interruptions repeated to it, raises its wipers to the desired level, and rotates them and selects an idle trunk in the usual manner.

The calling subscriber may now operate his calling device in accordance with the remaining digits of the desired number, whereupon the connection is extended to the desired line in the usual manner;

I After the called subscriber has been signalled and has responded, the two subscrib- 129 of the normally coners may converse with each other as desired, the talking circuits that are shown being outlined by the heavy conductors.

After the conversation is finished thetwo subscribers replace their receivers. When the receiver is replaced at the calling substation the circuit of the line relay 100 of the repeater. E is opened. Line relay 100, upon deenergizing, opens the bridge across conductors 113 and 114 at armature 106, whereupon: the connection in the distant exchange is released in the usual manner. As a further result of the deenergization of line relay 100 it opens the circuit of release relay 101 at armature 105. Release relay 101, upon deenergizing, removes the ground potential from release trunk, conductor 73 at armature 107, whereupon'gswitching relay 49 of the selector D and switchin relay 14 of the line switch 0 deenergize. he deenergization of switching relay 14 restores conditions in the line switch C to normal, and the deenergization of switching relay 49 of the selector D completes a circuit for release magnet 50 at armature 64, which circuit includes armatures 55 and 57 and off-normal contacts 51. By the operation of release magnet 50 the shaft wipers of the selector D are restored to normal position in the usual'ma-nner.

It may be said that, in case one of the one-way trunks has become idle in the meantime, in which case the ground potential will have been removed from conductor 84, relay 87 deenergizes when the round potential is removed from conductor 43 and replaces the ground potential upon release trunk conductor 73" at armature 86. If, however, the one-way, trunks are still all busy, relay 87 is maintained energized and the removal of the ground potential from conductor 73 renders repeater E accessible again. Similarly, if one or more of the in dividual trunks to which the line switch C.

and others in the same group have access has become idle in the meantime, the ground potential will have been removed from conductor 39 and-relay 40 deenergizes in response to the removal of the ground potential from release trunk conductor 29, and replaces the ground potential upon release trunk conductor 29 at armature 42.

If, however, all the individual trunks are still busy, the. selector D is rendered ascessible as soon as the ground potential is removed from release trunk conductor 29.

It may be said at this time that the arrangement, shown in the drawings of having a number of one-way trlmks normally accessible in the banks of an ofiice selector such as the selector D anda number of twoway trunks which are used only after all the one-way trunks have become busy, is an especially well suited adaptation of the arrangement, of having two groups of individual trunks and one group of common trunks, inasmuch as it occurs many times that the peak of the traffic from an exchange to a second exchange often occurs an hour or two sooner or later than that peak of the traffic from the second exchange to the first exchange. When such is the case, the common two-way tiunks areseldom called upon to carry traflic in more than one direction at a time and consequently in many cases they need be only large enough totake care of the difference between the average and the peak load from either of the two exchanges, from which they are accessible, to the other exchange.

It will be understood that when the number of trunks makes it necessaryithe one-way trunks leading from either exchange to the other may be split into as many subgroups as desired and that the group of two-way trunks may be common to all the groups of switches from which said one-way trunks are accessible, giving an arrangement similar tothe one shown in connection with the trunks accessible to the rotary line switches, Fi l.

%Vhile the explanation might be still further amplified, it is thought that what has been said already is sufficient to enable the invention to be thoroughly understood by those skilled in the art and to whom this specification is directed, and the various features of the invention, having been explained and described thus, will now be pointed out in the appended claims.

I claim:

1. In a telephone system, twoexchanges, one-way trunks leading from each exchange to the other, automatic switches in each exchange for seizing the said one-way trunks that lead from that exchange to the other, a group of two-way trunks accessible to the said automatic switches in both exchan es but normally made busy, and means for rendering said two-way trunks idle to the said switches in one exchange only, while all the one-way trunks leading from that exchange to the. other are busy.

2. In a telephone system, two exchanges, a group of two-way trunks connecting said exchanges and normally maintained in an artificially busy condition at both ends, a group of one-way trunks extending from the first exchange to the second exchange and a similar group of trunks extending from the second exchange to the first, and means for removing the artificial busy condition of said two-way trunks at the first exchange while maintaining it at the second exchange in case all of the said first group of one-way trunks become busy.

3. In a telephone system, a plurality of groups of automatic switches, a group of trunks accessible to said switches and divided int-o sub-groups, there being one subgroup individual to each group of switches, a group of overflow trunks accessible to all of said groups of switches in common but normally made busy, a plurality of relays individual to each overflow trunk, one of said relays for each group of switches, and means including the relays associated with any particular group of switches for rendering the overflow trunks idle at such group of switches whenever all the trunks of the associated individual subgroup become busy.

4. In a telephone system, a plurality of groups of automatic switches, a group of trunks accessible to said switches and divided into sub-groups, there being one subgroup individual to each group of switches, a group of overflow trunks accessible to all of said groups of switches in common but normally made busy, a plurality of relays individual to each overflow trunk, one of said relays, for each group of switches, and means including the relays associated with any particular group of switches for rendering the overflow trunks idle at such group of switches only when all the trunks gt the associated individual sub-group are usy.

5. In a telephone. system, a plurality of groups of automatic switches, a group of trunk lines accessible to and individual to each group of switches, a group of overflow trunks accessible to all of said switches in common, a test conductor individual to each overflow trunk, and a branch of said test conductor individual to each group of switches and normally disconnected from said test conductor.

6. In a telephone system, a plurality of groups of automatic switches, a group of trunk lines accessible to and individual to each group of switches, a group of overflow trunks accessible to all of said switches in common, a test conductor for each overflow trunk, a branch of said test conductor individual to each group of switches and normally disconnected from said test conductor, and means for separately connecting any branch to its associated conductor while maintaining the other branches associated with such conductor disconnected.

7. In a telephone system, a plurality of groups of automatic switches, a group of trunk lines accessible to and individual to each group of switches, a roup of overflow trunks accessible to all 0 said switches in com-mon, a test conductor for each overflow trunk, a branch of said test conductor individual to each groupv of switches and normally disconnected from said test conductor, and means whereby all of said individual branches are normally connected to a source of test potential to make the overflow trunks busy.

8. In a telephone system, a plurality of groups of automatic switches, a group of trunk lines accessible to and individual to each group of switches, a group of overflow trunks accessible to all of said switches in common, a test conductor for each overflow trunk, a branch of said test conductor individual to each group of switches and normally disconnected from said test conductor, said branches being all normally grounded, and individual relays for separately disconnecting said branches from ground and for connecting them with the associated test conductors.

9. In a telephone system, a plurality of groups of automatic switches, a group of trunk lines accessible to and individual to each group of switches, a group of overflow trunks accessible to all of said switches in common, a test conductor for each overflow trunk, a branch of said test conductor individual to each group of switches and normally disconnected from said test conductor, a relay for connecting each branch with its associated test conductor, and means controlled by the busy condition of the trunks individual to a group of switches for operating the relays at such group of switches to connect the associated branches trunks individual to a group of switchesfor operating the relays at such group of switches to connect the associated branches with said test conductors, and means for maintaining any operated relay energized independent of the busy or idle condition of the trunks individual to the associated group of switches. I

11. In a trunking system, an overflow trunk accessible in common to a plurality of groups of switches, a test conductor individual to said trunk, and branches of said test conductor individual to said groups of switches and normally disconnected from said test conductor.

12. In a trunking system, an overflow trunk accessible in common to a plurality of groups of switches, a test conductor for said trunk, branches of said test conductor individual to said groups of switches and normally disconnected from said test conductor, said branches being normally connected to a source of busy pouctor, means tential, and individual relays for separately disconnecting said branches from said source and for connecting them to said test conductor.

13. In a trunking system, an overflow trunk accessible in common to a plurality of groups of switches, a test conductor for said trunk, branches of said test conductor individual to said groups of switches and normally disconnected from said test conductor, an individual relay for connecting up the proper branch when the said overflow trunk is to be seized from a particular group of said switches, and a circuit including said test conductor for maintaining said relay energized as long as the overflow trunk is in use.

14. The combination, with an overflow trunk, of a release trunk conductor comprising a guarding section and a holding section, the guarding section being normally connected to ground, a relay energized by means outside the trunk for uniting said sections and for disconnecting the guarding section from ground, and a circuit including the holding section for maintaining said relay energized.

15. In a telephone system, a plurality of groups of switches, a trunk accessible in common to all of said groups comprising a combined test and holding conductor through the medium of which the trunk is normally made busy artificially, other trunks accessible to each of said groups of switches and individual thereto, and means for rendering said common trunk idle to the switches of any of said groups only ]when all the trunks individual thereto are usy.

16. In a telephone s stem, a trunk normally made busy artlficially, said trunk comprising in addition to the one or more talking conductors only a single other conductor, a plurality of groups of switches to which said trunk is accessible in common, other trunks accessible to each of said groups of switches and individual thereto, and means for rendering said common trunk idle to the switches of any of said groups when all the trunks individual thereto are busy.

17. In a telephone system, first and second exchanges, a group of switches in the first exchange, a group of one-way trunks accessible to said switches and extending to the second exchange, and a group of twoway trunks extendlng between said exchanges and accessible to said switches only when said one-way trunks are busy.

18. In a telephone system, first and second groups of automatic switches located in first and second exchanges, respectively, a group of one-way trunks accessible to the switches of said first group and extending to the said second exchange, a second group to the second group of switches only when of one-way trunks accessible to the switches the said second group of one-way trunks of the second group and extending to the are all busy. 10 first exchange, and a group of two-way In Witness whereof, I hereunto subscribe 5 trunks accessible to said first group of my name this 21st day of Nov., A. D., 1921.

switches only when the said first group of one-way trunks are all busy and accessible CLARENCE E. LOMAX. 

